1. Field of the Invention
The present invention relates to homokinetic joints and in particular to joints which can slide very freely and which may be employed in high-speed transmissions.
2. Description of the Prior Art
Universal (or Hooke) joints mainly employed in motor vehicles are simple and reliable mechanical units having an excellent efficiency and are employed universally. However, the field of utilization of these mechanisms is limited by two well-known fundamental handicaps:
(a) The absence of homokinetic operation of the universal joint which creates torsional pulsations whose amplitude increases as the square of the operating angle.
(b) The opposition to sliding of the splined telescopic device usually associated with the universal joint to permit the adaptation of the shaft in length by compression and extension. While this sliding device may operate very smoothly when it is checked by hand, it is practically blocked in operation when a torque is applied thereto, as usually occurs in use.
It has been attempted to remedy these two major drawbacks of the universal joint by substituting therefor slidable homokinetic joints such as those employed in lateral transmissions of front wheel drive vehicles or those having a rear drive through independent wheels.
Unfortunately, the two families of homokinetic joints employed in these lateral transmissions, which are joints employing balls, or a tripod element, each have by their particular type of operation a limit in their use at high speed for the desired mean angles of operation. This limit may be characterized by the maximum possible product: .beta.N, in which .beta. represents the continuous operating angle and N the continuous working speed under the required conditions of long life, reliability and comfort. If .beta. is expressed in degrees and N in revolutions per minute, it is usually acknowledged by experience that the upper limit of the product .beta.N for a universal joint is in the range of 20 000 to 30 000 depending on the size. In this case, the limit is due to the pulsating torque resulting from the torsional acceleration and the lack of homokinetic operation proportional to the square of .beta.N.
In the case of joints employing balls, the limits are determined by their heating and their excessive rate of wear resulting from mechanical losses which are much higher than those of a universal joint, so that the allowable .beta.N is lower than that of the universal joint.
In the case of tripod joints which have an excellent efficiency and a sufficiently long life, the limit results from the planetary movement which is not permissible at very high speed.